In commonly owned U.S. Pat. Nos. 3,792,316 and 4,074,334 there has been disclosed and claimed a system of this type wherein a power transistor coacts with a protective circuit including a monitoring transistor connected across a resistance in an emitter lead of the power transistor. The collector/emitter path of the latter transistor is shunted by a voltage divider consisting of a resistor in series with a diode, the latter being connected in bucking relationship with the base/emitter path of the monitoring transistor. The monitoring transistor controls a switching transistor forming a bypass for signals fed to the power transistor whenever the collector current I.sub.c and the collector/emitter voltage V.sub.CE fall outside a predetermined safety area. This arrangement prevents the flow of excessive load currents (e.g. in the case of a short circuit) which could damage or destroy the final stage of the amplifier.
The response of such a protective circuit is virtually instantaneous. This, however, is not always desirable. With a purely or predominantly reactive load, e.g. a loudspeaker, the number of volt-amperes in the final stage may safely reach short-term values amounting to five or more times the maximum power that can be tolerated with purely resistive loads. On the other hand, a sharp reduction of the output current in the presence of a brief overload not only leads to signal distortions, which may involve the generation of jarring noises in audio transmission, but may also result in the emission of high-frequency transients interfering with electronic equipment close by.
In order to avoid these inconveniences, it has already been proposed to provide means in such integrated circuitry to retard the response of the protective device or system by a delay on the order of one millisecond, sufficient to outlast peak amplitudes which in the case of audio signals endure only for intervals on the order of tens of microseconds. Such conventional delay means may take the form of a supplemental condenser which, because of its high capacitance (on the order of tenths of microfarads), cannot be incorporated in the integrated circuit and thus constitutes a separate component, requiring additional terminal areas on the IC chip and associated binding posts on its container whereby both the costs and the dimensions of the unit are increased.